In recent years, various techniques have been developed at very low temperatures using liquefied gases, such as liquid helium or liquid nitrogen. For example, such techniques are used to prepare cryogenic conditions for medical applications, such as superconducting quantum interference device (SQUID), magnetic resonance imaging (MRI), and superconducting magnetic energy storage (SMES). Liquefied gases are also used in the transportation field, such as linear motor cars, as well as aeronautical and space applications.
For these applications, various materials have been proposed. For example, they are organic materials such as fiber-reinforced plastic materials, ceramics and metal materials (e.g., stainless steel, aluminum alloy). These materials are used for various cryostats, containers (e.g., Dewar's vessel), supporting materials and the like.
Cryostats are required to have non-magnetic properties, low electrical conductivity, vibration-damping properties, low thermal conductivity, low He-leaking properties and the like. Containers are required to have low He-leaking properties and the like. Supporting materials are required to have dimensional stability, low thermal conductivity and the like. In addition to these properties, mechanical properties such as workability are also required.
In satisfying these properties, fiber-reinforced plastic materials, particularly those containing glass fibers, i.e., glass fiber-reinforced plastic materials (GFRP), are excellent in electric and magnetic properties, mechanical properties and workability, and have been widely employed. The glass fiber-reinforced plastic materials are formed into various members, depending upon the applications thereof, such as tubes, bars, and plates.
Examples of these known members of glass fiber-reinforced plastic materials are illustrated by using the accompanying drawings.
FIGS. 10a and 10b show a tube-shaped member made of a conventional glass fiber-reinforced plastic material. The tube-shaped member is produced by winding glass fibers 1 in the form of a multilayer coil by the filament winding (FW) method and binding it with epoxy resin 2 as a matrix by setting. The use of an epoxy resin ensures that the glass fiber-reinforced plastic material will have a satisfactory strength, based on the strength of glass fibers, even at low temperatures. When a vinyl ester resin is used in place of an epoxy resin, the resin can also be set at room temperature.
FIGS. 11a and 11b show a bar-shaped member made of a conventional glass fiber-reinforced plastic material. The bar-shaped member is produced by pultrusion of glass fibers 1 using epoxy resin 2 as a matrix. In this case, the mechanical strength of the member can be improved by increasing the content of glass fibers and drawing it in a smaller diameter.
FIG. 12 shows a plate-shaped member made of a conventional glass fiber-reinforced plastic material. The member is produced as follows: glass fibers are woven into fabric 3, and a plurality of such fabrics are successively stacked, while being bound together with an epoxy resin, to form a plate-shaped member. In this case, the degree of contraction at low temperatures varies with a change in the weight ratio of glass fibers and epoxy resin, and it is, therefore, necessary to prepare a plate-shaped member at the predetermined weight ratio.
However, these known member of conventional fiber-reinforced materials have inferior dimensional stability. That is, glass fiber-reinforced plastic materials used for these members have a tendency to contract gradually with a decrease in temperature during the use. Even if the member is adequately positioned at room temperature, a problem is caused at low temperatures in that the member will shift its position because of the thermal contraction ocurring with a decrease in temperature.
For example, in the case of a tube-shaped member as shown in FIGS. 10a and 10b, even if the tube-shaped member is adequately positioned at room temperature, the member may contract both in the radial and axial directions at low temperatures, so that, when another member is provided on the periphery of the tube-shaped member, a gap may be formed therebetween, and when one end of the tube-shaped member is fixed, the member may move to the side of the fixed end.
Moreover, in cases where a bar-shaped member as shown in FIGS. 11a and 11b is used to support another member under fixed tensile, the axial contraction of the bar-shaped member may decrease the tensile with a decrease in temperature.
Further, in cases where a plate-shaped member as shown in FIG. 12 is inserted as a spacer between other members, even if the plate-shaped member satisfactorily serves as a spacer at room temperature, there may be formed a gap therebetween because the plate-shaped member contracts in the thickness direction with a decrease in temperature, so that the plate-shaped member cannot exhibit the function of serving as a spacer.